1. Field of the Invention
The present invention relates to an X-ray imaging apparatus.
2. Description of the Related Art
X-ray phase imaging is a method of acquiring information on a sample object by producing contrast on the basis of shifting the phase of X-rays. One of such X-ray phase imaging methods is Talbot interferometry.
Talbot interferometry requires at least the following components: an X-ray source that emits spatially coherent X-rays, a diffraction grating that periodically modulates the phase of the X-rays, and a detector. When spatially coherent X-rays are transmitted through the diffraction grating, the phase of the X-rays periodically changes in correspondence with the shape of the diffraction grating. This change forms an interference pattern called a self image at a certain distance called Talbot distance from the diffraction grating. A sample object is positioned between the X-ray source and the diffraction grating or between the diffraction grating and the detector. Since the shape and the refractive index of the sample object influence the self image, the self image is deformed. Information on the phase of X-rays transmitted through the sample object (hereinafter also referred to as phase information) can be acquired on the basis of the deformation of the self image. Exemplary phase information on the sample object include a differential phase image, a phase image, and a scattering image.
In general, the pitch of a self image formed in Talbot interferometry using X-rays is generally smaller than the pixel pitch of the detector. Therefore, it is difficult to directly detect the self image. Hence, an absorption grating is used. The absorption grating has a periodic structure in which shielding portions that block X-rays and transmissive portions that transmit X-rays are periodically arranged. That is, a moire pattern is formed by shielding some portions of the self image with the absorption grating; the resultant moire pattern is detected for phase retrieval. In this manner, phase information on the sample object is acquired.
To form a moire pattern by using the self image and the absorption grating, the shielding portions of the absorption grating need to block X-rays to such an extent that a moire pattern is formed, and the period of the shielding portions and the transmissive portions needs to be substantially the same as the period of the self image.
PCT Japanese Translation Patent Publication No. 2009-543080 (International Publication No. WO2008006470, counterpart to publication: US2009/0316857) discloses an X-ray imaging apparatus including an absorption grating in which structures made of a material, such as resist or silicon (Si), having a high transmittance to X-rays are provided and gaps between the structures are filled with gold plate.
Gold plating, which is employed in the structure disclosed by PCT Japanese Translation Patent Publication No. 2009-543080 (International Publication No. WO2008006470, counterpart to publication: US2009/0316857), is suitable for fabrication of an absorption grating including shielding portions that are made of gold and each have a high aspect ratio.
Nevertheless, in the fabrication of such a gold-plated absorption grating, chipping of the mold and deposition of plating metal on the surface of the mold may occur, leading to irregularities in the period of the periodic structure. If the periodic structure includes any regions having irregularities in the period thereof, a desired moire pattern cannot be formed. That is, an imaging area may include portions from which phase information on the sample object is difficult to acquire through the detection of deformation in the self image of the sample object.